The inkjet-recording method is a printing method of obtaining a print by causing droplets of an ink composition to fly and depositing the droplets on a recording medium, such as paper. The method characteristically enables printing of a high-definition, high-quality image at high speed with a relatively inexpensive device.
Ink compositions for use in ink jet recording generally contain water as the principal component, as well as a coloring component and a moisturizer, such as glycerol, to prevent ink clogging and for other purposes. Water-soluble dyes have been widely used as the colorant for use in the inkjet-recording ink composition, because of the high chroma of colorant, the diversity of available colorants, solubility in water, and other factors.
However, dyes are often inferior in many properties, such as light resistance and water resistance, and thus, many prints printed by using a dye-based ink composition are inferior in light resistance and water resistance. As to water resistance, introduction of inkjet recording papers (papers designed exclusively for inkjet recording) having an ink-absorbing layer brought some improvements, but the ink printed on plain paper hardly has sufficient resistance to water yet.
On the other hand, pigments are superior in light resistance and water resistance compared with dyes, and use of a pigment as a colorant for inkjet-recording ink composition has been studied recently for improvement in light resistance and water resistance. Because pigments are generally insoluble in water, in order to use a pigment in a water-based ink composition, it is necessary to prepare the ink composition after mixing a pigment in a dispersant, such as a water-soluble resin, and stably dispersing the pigment in water.
It is necessary to study the kind and particle diameter of pigment, the kind of resin to be used, the dispersion means, and the like, to disperse the pigment stably in a water-based system, and there have been many dispersion methods and inkjet-recording inks studied.
For example, JP-A-8-3498 (“JP-A” means unexamined published Japanese patent application) describes use of a surface-modified carbon black in an ink, which carbon black is prepared by oxidation of an acidic carbon black with a hypohalogenous acid, and the thus-obtained pigment can be dispersed independently as it is. Alternatively, JP-A-10-120958 describes use, in an ink, of a surface-modified carbon black, prepared by oxidation of a carbon black, having a primary particle diameter of 40 nm or less and having a DBP (dibutyl phthalate) oil-adsorption amount of 60 mL/100 g, with a hypohalogenous acid. Use of such a surface-modified carbon black pigment described in these references, which is dispersible freely and independently, in an inkjet-recording ink is advantageous in obtaining a print with high color density on plain paper. However, it is necessary to take measures, for example, to increase the pigment concentration in ink, in order to achieve higher color density. Increasing the pigment concentration often makes it difficult to prepare stable ink with sufficient fluidity for use as an inkjet-recording ink. Alternatively, JP-A-2004-149765 discloses use, in inkjet ink, of a colorant, such as metal particles or alloy particles, enclosed in a dendrimer resin, to impart dispersibility of colorant, storage stability, particle-diameter uniformity, ink-ejection efficiency, and other properties. However, the method still had the problem that it gave an image having metallic gloss, leading to a sense of incompatibility, when a solid image was printed at high optical density.